Diode laser assemblies are employed in a variety of applications. Such assemblies include multiple diode array “bars” that are configured adjacent each other in an assembly. These assemblies are commonly referred to as stacked arrays. An example of such an assembly is provided in the perspective view of FIG. 1. In the assembly 20, a plurality of diode bars 22 are positioned adjacent each other. Between each bar is a structure 23 which provides a means of support as well as means of heat removal. Each diode emitter 24 in each diode bar 22 emits a nominal amount of power. Collectively, the individual diode emitters 24 and diode bars 22 in the assembly operate to provide a large aggregate power level that is useful for certain applications, including the pumping of larger lasers that require high power input pumping.
FIG. 2 is a perspective view of a typical diode bar unit. In this example, the diode bar 22 includes ten diode emitters 24. Each diode bar 22 is, for example, 150 microns wide, but emits an output beam 26 from an emitting surface of each emitter 24 that is only, for example, 1 micron wide and 100 microns long. The beams 26 emanating from the individual diode emitters 24 have a relatively broad angular divergence in one direction or axis and a relatively lesser degree of divergence in the orthogonal direction or axis (often referred to as the “fast” and “slow” axes). This pronounced difference in angular divergence is characteristic of the outputs of edge-emitting laser diodes. For example, in a first orthogonal direction along the x-axis of FIG. 2, the angular divergence θ1 of the output beam is 10°, and in a second orthogonal direction along the y-axis of FIG. 2, the angular divergence θ2 of the output beam is 30°. A cylindrical lens is commonly used to collimate the multiple beams emitted by the diode bar, in order to control the divergence in the y-axis direction. For some cases, lenslet arrays can be positioned over the string of emitters to collimate both axes. In conventional embodiments, such lasers are, for example, based on InGaAs on a GaAs substrate, emitting at a wavelength of 940 nm or based on InGaAlAs on a GaAs substrate, emitting at a wavelength of 808 nm.
When a beam 26 emerges from the bar 22, the brightness B (Watts/cm2/ster) of an individual emitted beam is P1/(Ae*Ω), where P1 is the power of the individual emitter 24, Ω is the solid angle of the beam divergence, and Ae is the emitter area. In this example, Ω˜θ1*θ2*(π/180)2, and Ae˜1 micron*100 microns, where the use of “˜” herein means “approximately equal to”. When emitters are combined in a bar 22, and bars are stacked in an assembly 20, the effective brightness becomes N*P1/(Astack*Ω), where N is the total number of emitters in the bar times the number of bars in the array, and Astack is the area of the stacked arrays. The area of the stack is the length of the bar times the separation between bars times the number of bars in the stack. The separation between bars is the width of the bar plus the width of the structure required for support and cooling, and is commonly on the order of 2 mm to 3 mm. The solid angle Ω of the stack emission is approximately the same as that of the individual emitters. The corresponding average intensity I (Watts/cm2) is N*P1/(Astack).
The brightness of the stack is limited by the space required between emitters in the bar and the space between bars. This space is determined by the requirement to remove waste heat from the emitters. Without proper heat distribution and thermal gradient control, the lifetime of the components are shortened, and the wavelengths of the emitted beams are more likely to vary over time. Currently, in systems that employ continuous wave (CW) diode bars, power densities on the order of 200 W/cm2 can be achieved, but this power density is currently limited to this amount, since the diode bars cannot be placed much closer together than 3 mm (assuming a 60 W diode array), due to heat distribution concerns.
In order to achieve higher power density in the output plane, others have resorted to the use of silicon monolithic microchannel laser (SIMM)-based laser arrays. SIMM-based technology improves on the removal of waste heat from the assembly, so that the density of diode emitters, and the proportion of emitting area relative to non-emitting area is increased. Such devices are capable of achieving power densities on the order of 600 W/cm2, or about three times higher than that of current diode bar assemblies, and utilize a lens array structure that provides a much lower net divergence of 0.6° along the x-axis and 60 along the y-axis. However, SIMM-based technology is much more expensive, and therefore is limited in practical use.
Others have employed stacks of interleaved optical plates to combine the outputs of two laser assemblies. One example of this is disclosed in Leibriech et al. “Powering Brightness”, SPIE OE Magazine, September 2001, pp 18-19. In this example, the output beams of left and right laser diode bars, or “stacks” are coupled into respective parallel plates, each having the height of half the pitch of each stack. The parallel plates interleave the beams, to thereby increase the brightness, and therefore the power density, of the resulting, interleaved output beam. However, this approach requires a collimation lens to transfer the output beams of the laser stacks into each respective plate. Collimation lenses are required because the bar output must enter the plate at an angle thus resulting in a large distance between the bar and the plate at one end of the bar. Without the collimation lens, the large divergence angle of the beam would result in most of the light emitted from the side farthest from the plate missing the entrance face. The angled entrance to the plate is required because this method of interleaving relies on refraction of the beams through the plates to produce the merged output beam. In many applications, the addition of the collimation lenses adds extra complexity and cost. If broad divergence was required, additional lenses would need to be placed at the output of the plates.